This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 10-171522, filed Jun. 18, 1998; and No. 10-220340, filed Aug. 4, 1998, the entire contents of which are incorporated herein by reference.
The present invention relates to a transparent touch panel capable of preventing the visual recognizing capability by the reflection of the external light from being lowered when the transparent touch panel is mounted to a liquid crystal cell, and to a liquid crystal display device equipped with the transparent touch panel.
In recent years, liquid crystal display elements attract attentions as an image display element. The liquid crystal display element is expected to be applied to, for example, a portable data assist notebook, an information terminal, a viewing finder for a video camera and a monitor for a car navigation system. Also, in recent years, desired is an input system in which a transparent touch panel is disposed on a display element for performing the input operation while observing the display screen. Known is a tough panel of a resistance film type. In the conventional tough panel of a resistance film type, a touch panel prepared by using polyethylene terephthalate (PET) film having a transparent conductive film formed thereon and a glass plate having a transparent conductive film formed thereon is superposed on a liquid crystal display element.
On the other hand, with expansion in the use of the touch panel, required are an improved contrast of the display and the function of preventing reflection of the external light. To meet these requirements, it is attempted to make the touch panel and the liquid crystal display device integral. For example, proposed Hin Japanese Patent Disclosure (Kokai) No. 10-48625 is a liquid crystal display device. It is proposed that a touch panel is arranged between a polarizing plate on the display side and a liquid crystal cell, said liquid crystal cell having a liquid crystal material sealed between transparent electrode substrates. Also, a retardation corresponding to xc2xc wavelength of the visible light is imparted to the transparent conductive substrate included in the touch panel. Alternatively, a retardation film is bonded to the transparent conductive substrate so as to prevent the internally reflected light produced by incorporation of the touch panel.
It is important for the retardation film to have a uniform retardation because the optical characteristics of the retardation film affect the polarized state of the light. The PET film used in the past as the touch panel is a crystalline plastic film and, thus, is partially crystallized within the film. As a result, the molecules are nonuniformly oriented, leading to a large unevenness in the retardation. It follows that the PET film cannot be used for the purpose described above. Therefore, it is studied to use a film made of an amorphous plastic material such as polycarbonate, polyarylate, polysulfone, polyether sulfone or a modified polyolefin having a norbornene skeleton.
The touch panel using any of these films permits achieving a desired improvement in the characteristics when viewed in a direction perpendicular to the display surface. To be more specific, the contrast is improved and the reflected light is suppressed, compared with the case where the touch panel is combined above the liquid crystal display device, so as to provide a display screen that can be observed very easily. However, if the display surface is observed obliquely during the color display or graduated display, the graduation is inverted compared with the displayed image of the liquid crystal display device before integration of the touch panel. In the extreme case, the monochromatic display is inverted or the color balance of the color display is collapsed so as to bring about practical problems such as deterioration in the quality of the displayed image and the poor viewing angle characteristics of the displayed image.
As a result of an extensive research conducted in an attempt to overcome the above-noted problems, the present inventors have found that a difficulty resides in the optical characteristics of the retardation film used in the touch panel. It has been found that the problems noted above can be solved by using a retardation film whose optical characteristics have been controlled with a higher degree in place of the retardation film obtained by the ordinary uniaxial drawing, leading to the present invention.
According to a first aspect of the present invention, there is provided a transparent touch panel, comprising:
a pair of transparent conductive substrates each having a transparent conductive film formed on at least one surface and arranged such that the transparent conductive films of the transparent conductive substrates are allowed to face each other, the pair of transparent conductive substrates being brought into mutual contact upon depression of the transparent conductive substrate on the upper side so as to perform the position detection; and
a retardation film made of an amorphous plastic material, meeting the relationship nz greater than ny, and having a retardation, which is represented by (nxxe2x88x92ny)xc3x97d, falling within a range of between 90 nm and 200 nm, where d represents the thickness of the retardation film, nx represents the maximum refractive index within the plane of the retardation film, ny represents the refractive index in a direction perpendicular to nx, and nz represents the refractive index in a thickness direction of the retardation film.
In another touch panel of the present invention, the photoelasticity coefficient of the retardation film falls within a range of between 5xc3x9710xe2x88x9213 cm2/dyne and 65xc3x9710xe2x88x9213 cm2/dyne.
According to a second aspect of the present invention, there is provided a liquid crystal display device integral with the touch panel of the present invention, comprising a polarizing plate arranged on the side of the display surface, a liquid crystal cell, and the transparent touch panel arranged between the polarizing plate and the liquid crystal cell.
According to a third aspect of the present invention, there is provided a liquid crystal display device integral with the touch panel of the present invention, comprising a polarizing plate, a first retardation film, a touch panel, a second retardation film and a liquid crystal cell, which are arranged in the order mentioned,
wherein, the touch panel is a resistance film type transparent touch panel,
the retardation value represented by (nxxe2x88x92ny)xc3x97d falls within a range of between 90 nm and 200 nm, where nx represents the refractive index in the direction of the slow axis (maximum refractive index) within the planes of the first and second retardation films, ny represents the refractive index in a direction perpendicular to the slow axis, nz represents the refractive index in the thickness direction of the retardation films, and d represents the thickness of the first and second retardation films, and
at least one of the first and second retardation films meets the relationship nz greater than ny.
In the liquid crystal display device integral with the touch panel of the present invention, the first retardation film meets the relationship nzxe2x89xa6ny and the second retardation film meets the relationship nz greater than ny.
Also, in the liquid crystal display device integral with the touch panel of the present invention, the first and second retardation films are arranged such that the nx directions of these retardation films are substantially perpendicular to each other.
Further, in the liquid crystal display device integral with the touch panel of the present invention, the first retardation film is made of a resin having a photoelasticity coefficient falling within a range of between 5xc3x9710xe2x88x9213 cm2/dyne and 65xc3x9710xe2x88x9213 cm2/dyne.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.